From the Drawing i pp ri pff
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The engineering staffs of DAF and Conoco put their brains togeth in order to work out how to remount an eight-year-old lpg tank ( a new chassis. They combined 'suck it and see' methods wi computer technology.
°REMOUNTING an eight-year-old lpg tank on a new chassis proved an interesting design exercise for the combined engineering brains of OAF and Conoco. The fuel company had an old AEC six-wheeler which was fitted with an lpg carrying pressure tank and while the chassis was nearing the end of its useful working life, the tank still had a lot of use in it. So it was decided to transfer the old tank on to a new chassis.
As with most things in life, it was not quite that simple. Nevertheless the resulting vehicle was a classic example of traditional "suck it and see" engineering linked with modern computer technology.
The tank, being built for carrying lpg, was a pressure vessel made from high-impact mild steel by Euroweld in Belfast. Because of its pressure vessel characteristics, any modifications necessary to adapt the tank to its new home or a DAF FAS 2105 chassis had to be restricted to the tank mountings. It was not practical to consider altering the tank itself in any way as this would have involved heat treatment to stress-relieve the modification and resubmitting it for testing and insurance validation.
The problem with installations such as this is that correct positioning is absolutely vital both from the axle loading point-ofview and also to obtain a satisfactory vehicle ride. To illustrate how critical it is DAF requires that the installation of a tank on an Eindhoven chassis be vetted by the company first even if it is for a repeat order.
A computer model was thus set up at Eidnhoven to show up the magnitude of any chassis • deflection with the natural frequencies of the cab and the springs to avoid the problem of cab nod when on the road. The advantage of using the dreaded computer of course is that it can be programmed with a number of variables to carry out complete "tests" in literally minutes saving a lot of time and effort. The computer cannot replace the original thinking of an engineer but it can certainly help with a lot of the routine calculations. The results frorn the computer program suggested that the centre line of the front mounting feet of the tank should be no further than one metre from the centre line of the front axle. With the combination of the Petroleum Regulations firescreen and the large spherical end of the tank, the original tank feet were a lot further than one metre from the front axle so the front feet had extensions welded on.
Although the exact centre line was still not quite within the one metre requirement, a lot of the foot assembly was, so the layout was judged to be acceptable.
When the computer had come up with its "recommendations" a chassis to this specification was tested complete with full instrumentation to measure the cab nod and the chassis deflection and the results tied in extremely well with the computer predictions.
Of course, it wasn't quite that easy as the wheelbase suggested by the computer as giving the optimum ride with the I pg tank came out at 3.97m (13ft) which was not a standard DAF wheelbase. Thus DAF took the nearest standard wheelbase of 4.5m (14ft 10in) and shortened the chassis. DAF prefers to shorten a long chassis rather than extend a short one because the latter method often sets up some doubtful points of high stress along the chassis. Shortening a long chassis does not have such unwelcome side effects.
At the back of the tank, the two rear feet come within the suggested design parameters with one over the front bogie axle and the other not far away from the trailing axle. The DAF engineers don't like having tank mounting feet too near to the extreme rear of the chassis as this can set up stress raisers in the side and cross members.
• The Conoco layout meant that the vibration from the road could be fed into the chassis with the minimum of stress.
The mounting feet should go down along the line of the frame section in theory, but because one of the main Conoco requirements was that the existing tank should be used without major modification, this entailed having mounting brackets with an overhang of about 50mm on either side —as shown in the accompanying diagram. Keeper plates around each foot assembly stop it from moving around. The bars were fitted between the tank feet across the chassis and stitch welded giving in effect another cross-member.
Because the resultant vehicle was such a success for Conoco, DAF has introduced a 3.97m wheelbase as a standard option as it has shown itself to be ideal for other customers as well. When doing conversion work such as this DAF checks every individual chassis and not just one from the batch according to John Beveridge, DAF's sales engineering manager.
As a "belt and braces" exercise, each chassis gets another going over at Marlow on top of the inspection it gets at Eindho
yen. The UK end of the DAF operation is so strict about this that it has been known for an operator to get late delivery of his vehicle if John Beveridge has been out.
The Conoco DAF was based on an FAS 2105 DHR 472 chassis with all the mechanical specification being as standard. The chassis was supplied as a standard unit from Eindhoven to Marlow who did all the actual conversion work although the parent company of course provided all the computer assistance.
As the design was approved by DAF literally at the drawing board stage, all the normal warranty arrangements apply. In the case of a tank or an installation failure, the warranty arrangements are agreed by discussion. But, claims John Beveridge "this has never happened anyway."
DAF will do this sort of conversion work for the owner-driver on a one-off basis as well as for the larger fleet operator. The customer puts in a request to DAF who then go into all the engineering ramifications before giving factory approval and tailoring the vehicles. The whole exercise is costed on the basis of materials and labour only with all the consultancy side of things being considered as part of DAF's overheads.
Although not part of the original exercise, the silencer used on the Conoco DAF is an interesting design. Known as the Cowl Spiral Silencer, it is of Canadian manufacture and is very small indeed, which makes it of particular interest because Pet Reg vehicles usually have their silencer slung across the front of the chassis. The snag is that it costs £140 compared to the standard DAF component at £89.94.